CN1328001C - Laser machining method - Google Patents

Abstract

The invention relates to a laser machining method for rapidly drilling holes in dielectric substrates (6). A Q-switched CO2 laser is used as a laser light source whereby generating a pulsed laser beam (4) with a pulse repetition frequency greater 50 kHz, with pulse durations shorter than 200 ns and with an energy per laser pulse of at least 10-4 joules. The laser beam (4) is directed onto substrate (6) to be machined by means of a diverting unit. The cited parameters that characterize the laser beam (4) ensure both a high production rate of drilled holes (5) as well as a high hole quality. For example, 500 holes per second can be drilled in a 0.4 mm-thick LCP substrate, whereby the shape of the drilled holes is approximately cylindrical or conical. The high production rate and the simultaneously high hole quality are a consequence of the selected wavelength, the short pulse duration, the high repetition rate, and of the pulse energy which is high compared to that of conventional laser machining devices used for manufacturing electronics.

Description

Laser processing method

technical field

The invention relates to a laser processing method for rapidly drilling small holes on a dielectric substrate.

Background technique

Material processing by means of laser beams has become increasingly important in recent years due to the rapid development of laser technology. Especially in the field of electronic product processing, laser processing of circuit boards and substrates has become an indispensable tool due to the increasingly miniaturized structural components. Generally, the required structural components and/or substrates can be realized according to the miniaturization of structural components. miniaturization. For example, small holes can be drilled in the substrate, which have a smaller diameter than the diameter of small holes drilled by conventional drills. The precondition is that the laser power of the laser beam generated on the substrate is known precisely. In addition, through-holes or so-called blind holes can also be drilled. Blind holes are especially important for multilayer circuit boards, because through The metallization can electrically connect the different metal layers of the multilayer printed circuit board to one another and thus significantly increase the integration density on the substrate.

A laser processing device is known from US 5,593,606, by means of which small holes with a diameter between 50 and 200 μm can be drilled in multilayer substrates. A continuously pumped good-quality Nd:YAG laser is used as laser light source, which generates laser pulses in the ultraviolet spectral region according to a frequency conversion. Each laser pulse has an average pulse power of approximately 250 mW and a pulse length of the order of 100 ns. Each pulse thus yields a very small energy of about 25 nJ, so that many laser pulses must be used to drill a hole. In addition, because the pulse repetition frequency is limited to a few kHz, the throughput, that is, the number of small holes drilled per unit time is correspondingly small, so only one per unit time can be drilled with this laser processing device depending on the material and the layer thickness to be drilled. relatively few pinholes.

It is also known that for material processing and especially for drilling small holes in substrates _CO2 lasers with a wavelength of 9.2-10.6 μm or pulsed solid-state lasers with a reference wavelength of 1064 nm, such as Nd:YAG lasers or Nd : YVO ₄ laser. The drawback of using such common _CO2 laser sources obtained in the infrared spectral region is that the pulse length of the generated laser pulses is relatively long in the order of μs. This puts the substrate to be processed under high thermal load, so that the geometry of the drilled holes deviates significantly from the optimum (cylindrical or cylindrical shape) due to drilling burrs or due to deposits on the edge of the hole. ) shape and thus reduces the quality of the drilled hole. The deposition occurs, for example, from solidified vapors which sublimate the substrate material due to the heating of the laser beam. However, deposition can also occur due to small particles of solid substrate material which are thrown onto the hole edges due to the strongly uneven heating of the substrate.

From DE 100 20 559 a method is known for processing materials by means of ultrashort laser pulses in the visible or near infrared spectral region. In this case, laser pulses with a pulse length of less than 300 ps and a repetition rate of 100 kHz to 1 GHz are generated by a laser source. The intensity of the individual laser pulses is amplified by the total generated laser pulses in an optical amplifier. The amplified laser pulse has a pulse length of less than 300 ps and a repetition rate of 1 Hz to 1 MHz, which is used for material processing. The unamplified pulses, which are also directed towards the material to be processed, are used for checking the material to be processed. For example, so-called optical coherence radiography is used as an inspection method.

Contents of the invention

The object of the present invention is to propose a method for drilling small holes in a dielectric substrate, in which a large number of high-quality small holes can be drilled in a short time.

The technical solution of the present invention is to provide a kind of laser processing method that is used for fast drilling on the dielectric substrate, uses a high-quality _CO2laser as laser source, and it produces a pulsed laser beam, and this laser beam has more than With a pulse repetition frequency of 50 kHz, a pulse length shorter than 200 ns and an energy of at least 10 ⁻⁴ Joules per laser pulse, the laser beam is deflected by a deflection unit onto the substrate to be processed.

The basic idea of the invention is that, with suitable parameters, namely wavelength, pulse length, repetition rate and pulse energy of the laser beam used for processing, not only the throughput, i.e. the number of small holes drilled per unit time, but also the resulting hole quality. According to the invention, a high-quality CO ₂ laser is used for generating the pulsed laser beam. High-quality switching of _CO2 lasers can be achieved with a so-called acoustooptisch switch. For example, a CdTe crystal is suitable for this, which is excited by a mechanical oscillation at a frequency in the megahertz region.

The processed laser beam according to claim 4 can in particular be focused to a diameter of 50 to 200 μm, if the laser beam emitted by the laser source is enlarged by a beam expander in front of its own focusing lens. In this regard, it should be pointed out that the use of beam expansion results in a smaller depth of focus of the processed laser beam, so that the distance between the focusing lens and the surface of the object to be processed must be maintained with the highest possible precision. In this way, undesired widening or conical geometries of the drilled holes can be avoided.

Drilling blind vias is especially used in the processing of multilayer substrates.

Depending on the substrate material and thickness or the depth to be drilled, the holes are drilled either by a single laser pulse or by consecutive laser pulses directed at the object to be processed. When using several laser pulses in succession, care must be taken that the individual laser pulses are generated as far as possible at the same position on the object in order to avoid poor drilling quality. Experiments already carried out have shown that the spatial overlap of the focal planes required for this remains at least 66%.

It is best to use pulses with a length of up to 150ns for processing substrates made of LCP ( Liquid Cristalline Polymer ), that is, liquid crystal polymer substrate materials. This substrate material has outstanding electrical properties up to 40GHz and it Not only impermeable to water, oxygen but also other gases and liquids.

For the processing of dielectric substrates FR4 (FlameRetard 4) mechanically reinforced by glass fiber materials, such as the material C-1080 of the company ISOLA, a pulse repetition frequency of at least 50 kHz is suitable according to claim 8, and pulses of 60 kHz to 100 kHz are especially preferred repeat frequency.

Description of drawings

Further advantages and features of the invention emerge from the following exemplary description of presently preferred embodiments. in the attached picture

Figure 1 schematically shows through-substrate drilling,

FIG. 2 graphically shows the maximum achievable drilling throughput as a function of the thickness of the LCP substrate used.

Detailed ways

According to the embodiment of the invention shown in Fig. 1, the drilling is realized by a laser processing device, and a laser beam with a wavelength of 9.2 ± 0.2 μm emitted by a not shown _CO2 laser passes through a beam expander 1 The diameter is expanded in such a way that the diameter of the laser beam is multiplied by a factor of 1.5 to 2 compared to the diameter of the laser beam at the output coupling mirror of the laser. The enlarged laser beam is deflected by 90° by a deflection unit 2 comprising at least two movable mirrors (not shown). The deflected laser beam is focused by a telecentric focusing lens 3 in such a way that a laser beam with an effective focus diameter of 100 μm to 200 μm is aimed at the object to be processed, which according to the exemplary embodiment shown here is a single-layer substrate Bottom 6. The two mirrors of the deflection unit 2 are arranged in such a way that the laser beam 4 can be aimed at the surface of the substrate 6 at any position within a given area within a short time.

The substrate 6 is made of LCP material. Such materials are offered, for example, as Vectra H840 from the company Ticona or Zenite 7738 from the company Dupont de Nemours. It is to be pointed out, however, that other dielectric materials such as FR4 or epoxy RCC can also be used for the substrate.

The laser beam 4 produced by a not shown high-quality _CO2 laser source emits laser pulses with a repetition rate of at least 50 kHz, which have a pulse length of less than 150 ns and a pulse energy of at least 0.7 mJ. The diameter of the laser beam 4 generated on the substrate 6 to be processed is 100 μm to 200 μm. Small holes 5 with a diameter of 120 μm to 250 μm are drilled in this way, the quality of the resulting holes being significantly improved compared to small holes drilled with conventional laser processing devices.

It should also be pointed out that in order to achieve a high drilling throughput it is necessary to configure the imaging unit 2 in such a way that the laser beam 4 deflected onto the substrate 6 to be processed can be deflected easily from a position on the substrate surface as precisely defined as possible. to another position on the substrate surface that is also defined as precisely as possible. Therefore, when using such a fast deflection unit 2, due to the relatively high pulse energy, due to the short pulse length and the wavelength of the laser beam 4 used, not only a high drilling throughput is achieved, but also the thermal load on the substrate material is minimized and Drilling burrs or deposits around the drilled holes are thereby avoided.

FIG. 2 shows the results of an experiment in which the maximum number of boreholes that can be drilled with high borehole quality is shown diagrammatically as a function of the substrate thickness. Here, the unit of the thickness d on the horizontal coordinate is millimeter (mm). The throughput N/t of boreholes drilled per second is given on the ordinate. The substrate used is still made of LCP material. The thinner the LCP substrate to be drilled, the higher the drilling throughput. 300 holes can be drilled per second for a thickness of 0.5mm. The number of holes that can be drilled with the same quality can be increased to 500 per second for a thickness of 0.4 mm. The throughput of drilled holes increased to 1250 holes per second at a thickness of 0.15mm.

It should also be pointed out that 0.4 mm LCP substrates consisting of so-called injection-molded substrates are frequently used in electronics processing, so that the throughput of 500 drilled holes per second is compared with the throughput achievable with conventional laser processing equipment obviously increase.

Overall, the invention implements a laser processing method for rapidly drilling small holes in a dielectric substrate 6 . As the laser source use a high-quality _CO2 laser that can generate a pulsed laser ray 4 with a pulse repetition rate greater than 50 kHz, a pulse length shorter than 200 ns and an energy of at least ^10-4 joules per laser pulse. The laser pulse 4 is deflected by the deflection unit onto the substrate 6 to be processed. By means of the above-mentioned parameters characterizing the laser beam 4 , not only a high throughput of drilled holes 5 but also a high drilling quality can be guaranteed. Therefore, 500 small holes can be drilled per second on a 0.4 mm thick LCP substrate, wherein the geometry of the drilled holes is approximately cylindrical or approximately conical. The selected wavelengths, short pulse lengths, high repetition rates and, compared to conventional laser processing devices, high pulse energies in the field of electronics processing result in high drilling quality at the same time as high throughput is achieved.

Claims (11)

1. A laser processing method for fast drilling on a dielectric substrate, characterized in that, · As laser source a high-quality _CO2 laser is used, which generates a pulsed laser beam (4) which - have a pulse repetition frequency greater than 50kHz, - have a pulse length shorter than 200ns and - having an energy of at least 10 ^-4 joules per laser pulse, • The laser beam (4) is deflected by a deflection unit (2) onto the substrate (6) to be processed. 2. Laser processing method according to claim 1, characterized in that a laser beam (4) with a wavelength in the range of 9.2±0.2 μm is used. 3. Laser processing method according to claim 1, characterized in that laser pulses with an energy of 0.7 millijoules are used. 4. The laser processing method according to claim 1, characterized in that the laser beam (4) is focused on the substrate (6) to be processed with a diameter of 50 μm-200 μm. 5. The laser processing method according to claim 1, characterized in that blind holes are drilled in the substrate (6) to be processed. 6. The laser processing method as claimed in claim 1, characterized in that a hole (5) is drilled by means of a single laser pulse or by means of a plurality of laser pulses which are aligned one behind the other on the substrate to be processed (6). 7. Laser processing method according to claim 1, characterized in that a liquid crystal polymer substrate is processed, wherein pulse lengths shorter than 150 ns are used. 8. Laser processing method according to claim 1, characterized in that a dielectric substrate mechanically reinforced with glass fiber material is processed, a pulse repetition frequency of at least 50 kHz is used. 9. The laser processing method according to claim 8, characterized in that a pulse repetition frequency of 60 kHz to 100 kHz is used. 10. Laser processing method according to claim 1, characterized in that an epoxy material is processed, a pulse repetition frequency of at least 80 kHz being used. 11. Laser processing method according to claim 10, characterized in that a pulse repetition frequency of approximately 100 kHz is used.